December 19, 2013

Detailing The Evolution Of Plumage Patterns In Male, Female Birds

Waterfowl such as ducks, geese and swans belong to the order Anseriformes. Game birds such as pheasants, partridges, hens and turkeys are known as the order Galliformes. The birds belonging to both of these orders are recognized not only for their meat, but also for the elegant display of their plumage.

Some members within the orders show differences between male and female, known as sexual dimorphism. Such as with the mallard, the male and female plumage is so different that for years they were thought to be a separate species altogether. However, in some species, various members of the same order show little difference between the two sexes.

Thanh-Lan Gluckman, a Cambridge PhD candidate, has researched this phenomenon and published her findings in the Biological Journal of the Linnean Society. She notes the similarities and differences in the plumage of almost 300 members of both orders - focusing mainly on the patterning between male and female instead of the color.

She said, “The color of plumage has attracted much research interest, but the exquisite patterns of bird plumage, such as the spots of the guinea fowl and the barred patterns of ducks and turkeys, to just name a few, have received much less attention.”

According to Gluckman, it has been argued male birds acquire their brilliant colors and exquisite patterning for mating and attracting females. Females kept a dull color to blend into the surroundings to protect their young, as well as themselves, from predators.

“My research looked at the plumage patterns of male and female birds on a separate and equal basis – and then went on to identify similarities and differences between them. By tracing the evolutionary pathways in the dimorphism of 288 species of waterfowl and game birds, I reconstructed the evolutionary history of plumage pattern sexual dimorphism, which allowed me to demonstrate that plumage patterns in females are not a result of genetic correlation," Gluckman said.

"Essentially, what I found was that plumage patterning is remarkably labile – both male and female birds have the capacity to change between different types of patterns. What’s interesting is to consider what are the forces driving these changes in male and female plumage patterns – whether they have an environmental basis and/or whether they have a signaling function between birds of different sexes or within the same sex,” she explained.

A paper by John Hunter in 1780, published in the Philosophical Transactions of the Royal Society of London, states the difference in plumage between male and female in a species of bird, are sexually driven. Since then, the view that the male shows off his coloring to win over a female has been understood. An explanation has been given to explain the dimorphism in correspondence with mating habits. A polygamous male (having more than one mate) will develop beautiful plumage to attract as many females as possible. On the other hand, a monogamous male (having only one mate) will have similar patterns and colors to the female.

But, this is not always true.

According to Gluckman, “Previous research has shown that the traditional argument that differences in plumage between the sexes stem from differences in breeding systems doesn’t always hold up. In many putatively monogamous species, the plumage of the males is significantly different to that of females and, likewise, males and females in many polygamous species have the same type of plumage. This suggests that plumage is not exclusively an outcome of breeding habits – but is a matter of function in a highly complex way.”

Gluckman studied the variations between the sexes within the same species and across species. This was to establish the similarities and differences between male and female plumage patterns and colors. She classified them under four categories: mottled, scaled, barred and spotted.

By concentrating on both the similarities as well as the differences in the plumage between male and female birds instead of if one sex is the same as the other -- she found there can be many different types of sexual dimorphism. She also found the changes in sexual dimorphism could stem from the changes in the male or the female themselves. It was also discovered the birds change easily into different types of dimorphism, depending on social and environmental conditions.